Tissue systems

Meristematic & permanent tissue

Primary growth (length) and secondary growth (width) occur from addition of new tissue via mitotic cell division

All plants have primary cell walls which are flexible and can expand as the cell grows
Some plants have secondary cell walls which contain lignin, are inflexible, and provide structural support
Meristematic tissue cells are totipotent (are able to differentiate into the entire variety of mature cell types) and are undifferentiated or incompletely differentiated
- Miotic cell division occurs here
- Apical, lateral, or intercalary
- Once formed, continue as meristematic or become permanent tissue
- Small cells with thin walls and dense cytoplasm with small vacuoles
- Large, dense nuclei contain chromatin and is located centrally
- Meristems can form from other injured cells & aid wound healing
1. Apical meristems or primary meristems give rise to primary plant body
  - located at tips of stems and roots enabling plant to extend in length
  - plant shoot system originates here as leaf and bud primordia, more complex than SAM
2. Lateral meristems are secondary meristems because they're responsible for secondary growth or increase in girth and thickness in a maturing plant
  - Not all plants exhibit secondary growth
3. Intercalary meristems occur at base of leaf blades and at nodes
  - Enables a leaf blade to increase in length from the base
Permanent tissue cells are no longer actively dividing but are differentiated into cells with distinct functions
- ground, epidermal, and vascular
- May be living or dead, thin or thick-walled
- Meristems produce cells that differentiate or specialize and become permanent to take on specific roles and can no longer divide
1. Dermal tissues cover and protect plant
  - Outermost covering
  - Water regulation, protection, gas exchange
  - Epidermis is a single layer of cells covering and protecting underlying tissue
  - Bark is a tough, waterproof outer layer of cork cells that protect woody plants from damage
  - Epidermal cells are the most numerous and least differentiated of the cells in the epidermis with thin walls, no chloroplasts, and alive at maturity. They secrete a waxy cuticle
    - The following features have specialized guard cells
      - Stomata are openings through which the uptake of carbon dioxide and release of oxygen and water vapor can occur on the epidermis of aerial ports of plant
        
        the guard cells on each stoma or pore shrink and swell to control exchange and water loss
        
        They have chloroplasts, are the only epidermal cells capable of photosynthesis
      - Trichomes are hair-like structures on surface of leaves and stems that develop from epidermal cells
        
        reduce loss of water from shoot system by decreasing flow of air over plant surface
        
        Can increase solar reflectance to protect against UV & overheating
        
        Glandular trichomes can store & exude compounds that defend against herbivory
      - Root hairs are tiny hair-like extensions of root epidermis that absorb water and mineral nutrients in the soil
        
        Increase root surface to access most amount of soil
2. Vascular tissue transports water, minerals, sugars to different parts
  - Consists of xylem and phloem - long distance transport
  - Lie adjacent to each other and are arranged in distinct strands called vascular bundles which run up and down the stem
  - The vascular bundle includes the phloem and xylem strands, meristematic tissue called vascular cambium sandwiched in between that creates new vascular tissue allowing secondary growth
  - Xylem conducts water and minerals from roots to leaves via transpiration (loss of water from stomata of plant)
    - Composed of tracheids, vessel elements, xylem parenchyma, and xylem fibers
    - Tracheids and vessel elements are dead at maturity & have thick cell wells of lignin, joined end to end to form hollow tube for water flow
      - pits in side of walls allow water to move from one cell to another
      - In growing tissue, lignified part of cell wall is in rings or helix allowing extension
      - In nonexpanding parts, cell wall forms as netlike or pitted structure for mechanical support
      - Xylem parenchyma are the only living cells in xylem involved in storage
      - Xylem fibers are sclerenchyma cells in between vessels and tracheids for mechanical support
  - Phloem tissue are composed of companion cells, sieve tube elements, phloem parenchyma, phloem slerenchyma
    - Living at maturity
    - Conduct dissolved sugars and organic molecules in process called translocation that transports products of photosynthesis to parts where energy is needed or stores
    - Sieve tubes are main conducting channels of phloem and contain cytoplasm but no nucleus
    - Companion cells are special type of parenchyma cell found alongside sieve tubes that provide energy used during translocation
    - Sieve tubes & companion cells not lignified
    - Pholem Parenchyma - storage
    - Phloem Sclerenchyma - mechanical support
3. Ground tissue serves as site for photosynthesis, provides a supporting matrix for vascular tissue, and helps store water and sugars
  - Derived from ground meristem tissue and makes up most of plant body
  - Metabolism, support and defense
  - Three types mainly differing on wall structure
  1. Ground Parenchyma cells are living cells making up bulk of soft internal tissue, commonly called pith
    - Thin, flexible cell walls
    - Large water filled vacuoles
    - Storage and metabolism
    - May contain sugars, organic acids, defense compounds, and starch
  2. Ground Collenchyma are living cells that are long and narrow with unevenly thick extensible cell walls composed of cellulose and pectin
    - Found in strands or sheets beneath epidermis (young stems, petioles, leaf veins)
    - Provide flexible support to actively growing regions
    - Sclerenchyma cells take over role of mechanical support when growth is complete
  3. Ground Sclerenchyma cells are typically dead, providing structural support and protection to non-growing regions
    - Cell walls are hard and rigid due to deposition of lignin
    - Classified as sclereids or fibers based on shape
      1. Fibers are elongated with tapering ends and often occur in bundles
        
        contribute more than half the volume of woody tissue and are found in all stems & leaves that are hard and tough
      2. Sclereids are shorter with more abundant and elaborate pitting
        
        Found in structures like seed coats, walnut shells, peach pits

	Created by Sophia Wright about 5 years ago

Note on Unit 4: Plant Anatomy, created by Sophia Wright on 21/09/2019.